1. Field of the Invention
This invention relates to moisture sensors and a process for the production thereof. This invention provides moisture sensors which have good resistance to water and resistance to chemicals, which also have good and stable performance, and which may be produced simply and conveniently.
2. Description of the Prior Art
Heretofore, as moisture sensitive substances in moisture sensors, there have been those employing ceramics, metal oxides, polyelectrolytes etc. Among those, it is known that moisture sensors employing a polyelectrolyte as a moisture sensitive substance and forming its films on various solid substrates (insulating materials, piezoelectric materials, semi-conductor materials etc.) are comparatively good in sensitivity and responsiveness.
Especially, there have been proposed various types of piezoelectric moisture sensors in which polyelectrolyte films are formed as a moisture sensitive substance on a piezoelectric element such as quartz crystal oscillator etc. and which determines the moisture utilizing as the index the change in the oscillating frequency of the quartz crystal oscillator based on the change in the weight due to the interaction (mainly hygroscopic action) between said polyelectrolyte and the moisture. The aforesaid moisture sensors are usually produced by forming first a pair of electrodes of an electrically conductive material, such as gold, silver etc., to cover a piezoelectric element plate by deposition and then forming on these electrodes a film of a polyelectrolyte which has been prepared beforehand (for example, styrene-divinylbenzene copolymer sulfonate, polystyrene sulfonate, mixtures of polystyrene sulfonate and polyvinyl alcohol, mixtures of polyvinyl alcohol and methycellulose sulfonate etc.) by such techniques as brush coating, spinner coating etc.
With the moisture sensors obtained by such a process, however, since the moisture sensitive film, i.e. the polyelectrolyte film, has been formed by coating, the adhesion with the electrode surface was not adequate, and in particular, on microanalyzing a trace water content, the moisture detecting performance was unstable and the responsiveness was also inadequate. Further, there has been a problem that the resistance to water and resistance to chemicals of the moisture sensitive film were not adequate and the use conditions was restricted thereby, and in addition, there was also a problem associated with the production that the control of the thickness of the moisture sensitive film was difficult.
On the other hand, there has also been known a moisture sensor in a form of an pair of electrodes on an insulating material and a polyelectrolyte film provided by coating between these electrodes. This moisture sensor is of a type for measuring the moisture based on the change in the electrical resistance or the electrical capacity by the water absorbed by the polyelectrolyte film, and also this type of the moisture sensor had problems with the polyelectrolyte film similar to those associated with aforesaid piezoelectric moisture sensors.
To solve the above-described problems associated with the moisture sensitive films in the moisture sensors, there have heretofore been made various proposals.
A first example is a proposal of moisture sensors of a resistance type (impedance detection) which improves the resistance to water and resistance to chemicals by forming a moisture-permeable protective polymer such as silicone resin on a polyelectrolyte film or by crosslinking a polyelectrolyte film either by ultraviolet irradiation or by employing a crosslinkable polymer [S. Miyoshi et al., "Thin-Film Humidity Sensors of Ion-Conductive Polymer", Sharp Technical Report, pp. 51-56, Vol. 22 (1982)]. However, such a moisture sensitive film leaves much to be desired in point of the responsiveness to the trace water content and the film thickness control.
Further, there has been proposed in Japanese Patent Application Laid-open No. 54840/1982 a piezoelectric moisture sensor which has an improved adhesion between a moisture sensitive film and a piezoelectric element plate by making said piezoelectric element plate porous.
Still further, there has also been proposed a moisture sensor of an electric capacity type which comprises a plasma polymerized polystyrene film formed as a moisture sensitive film between electrodes on an insulating substrate [S. Takeda, "Dielectric Properties of Polystyrene Thin Films Formed by rf Electrodeless Excitation", Journal of Applied Physics, pp. 5480-5481, No. 12, Vol. 47 (1976) and "Capacitive Humidity Element Using Polystyrene Thin Films by Plasma Polymerization", Journal of Applied Physics, pp. 1219-1224, No. 7, Vol. 20 (1981)]. However, the change in the capacity according to the change in the moisture was small, and also where the aforesaid film was applied to a piezoelectric moisture sensor, adequate response as moisture sensor was not obtained and thus unsatisfactory.
This invention has been made in order to eliminate the above-described various problems associated with the conventional methods. The present inventors have been intensively studying on this subject and, as a result, have discovered that by conducting plasma polymerization for the basic formation of a moisture sensitive film and further by incorporating hydrophilic groups into the plasma polymerized polymer film by chemical treatment, a moisture sensitive film which possesses excellent resistance to water, resistance to chemicals and adhesion coupled with excellent moisture responsiveness may be obtained, thereby having accomplished this invention.